This invention relates to centrifuges, and more particularly to apparatus for measuring the rotational velocity of an air driven centrifuge rotor.
Air driven centrifuges are used for centrifuging fluid mixtures of the type that require extremely high speed to separate into their respective fractions. Also, since it is generally true that at higher centrifugation speeds less time is required to achieve a desired fractionation, it has been recognized that air driven centrifuges are useful for reducing the processing time and cost of many types of laboratory work.
Air driven centrifuges such as disclosed in U.S. Pat. No. 3,958,753 issued to Durland et al., and assigned to Beckman Instruments, Inc., the assignee of the present invention, are capable of reaching speeds as high as 200,000 R.P.M. Speeds of this magnitude are attainable because the rotor of such a machine is rotated and supported on a cushion of air by pressurized air streams.
A problem which has been experienced by users of air driven centrifuges is that it has heretofore been difficult to ascertain true rotor velocity. Unlike shaft-driven centrifuges which can incorporate conventional tachometer means, the floating rotor of an air driven centrifuge is not adaptable to mechanical or electro-mechanical speed-sensing methods. Accordingly, users usually estimate rotor velocity by measuring the air pressure driving the rotor. Frequently, charts or graphs are employed to translate air pressure to R.P.M. (revolutions per minute). In many instances the accuracy of this method is found to be insufficient for the intended purpose. In such cases users have employed electronic photo-optical tachometers or stroboscopic speed measuring equipment to make more accurate determinations of rotor velocity. It is, of course, required that a window be provided on the machine to enable such optical means to measure rotor velocity. In many instances, the user has found these speed measuring methods manifestly inconvenient.
In the case of stroboscopic tachometers for example, it is necessary for the operator to adjust the flashing rate of the instrument until synchronism is achieved with the speed of the rotating object, at which point the motion of the object appears to freeze. The speed of the rotating object is then determined by reading the dial setting of the stroboscopic tachometer's flashing rate control knob. Accordingly, the speed readout by this method is not only not instantaneous, but the operator usually requires an assistant if it is desired to adjust the speed of the centrifuge, since the operator's two hands are occupied with holding and adjusting the stroboscopic tachometer.
In the case where handheld digital photo-optical tachometers are employed for measuring the rotor speed of an air driven centrifuge, it is generally necessary that the rotor carry a highly reflective target so that the photo sensor can detect and count each revolution of the rotor. It has been found extremely difficult to employ the usual devices such as reflective tape for this purpose, as the extremely high centrifugal forces tend to throw off such appliques. The necessity for providing a highly reflective target is in the main due to the relatively long distance between the external photo-optics and the rotor. As a matter of conjecture, there is also the possibility that a portable device may be inherently less sensitive (than a built-in tachometer) because the plane of the illuminator is likely to be parallel to the plane of the receptor, rather than convergent.